Carbonates such as calcium carbonate have long been widely used in areas such as rubber, plastics and papermaking. Recently, many types of carbonates with high functionality have been developed and put into practical use for a variety of applications and purposes according to particle shapes and diameters.
Examples of crystal forms of the carbonates include calcite, aragonite and vaterite. Of these, the aragonite is needle-like and has superior hardness and coefficient of elasticity, and it may be used for versatile applications.
Commonly known methods for producing the carbonates are, for example: (1) a reaction of a solution containing carbonate ions with a chloride solution, and (2) a reaction of chloride with carbon dioxide. Also, Patent Literature 1 proposes a method regarding the method (1) to produce a needle-like carbonate having aragonite structure, wherein a reaction of a solution containing carbonate ions with a chloride solution takes place under ultrasonic irradiation. However, the carbonate obtained by the production method described in Patent Literature 1 is not only oversized of 30 μm to 60 μm in length but also has a wide distribution in the particle size; a carbonate controlled for a desired particle size may not be obtained.
In addition, regarding a method of introducing carbon dioxide into water slurry of Ca(OH)2, Patent Literature 2 proposes a method to place a seed crystal of needle-like aragonite in Ca(OH)2 water slurry and grow the seed crystal only in a certain direction. Furthermore, there is a problem that only a large particle of 20 μm to 30 μm in length may be obtained even with the method for producing a carbonate described in Patent Literature 2.
On the other hand, in recent years, the majority of optical-glass products such as glass lens and camera lens have been replaced by polymeric optical materials in view of their lightness, easy processing, superior mass productivity, and simplicity in application of molding technologies such as injection molding and extrusion molding. For materials of optical components for opto-electronics, especially for optical components used for laser-related devices such as optical disc devices for recording sounds, images, literal information and the like, there is a strong tendency to use polymeric materials.
A polymeric material having birefringence does not cause a problem when it is applied to optical components which do not require relatively high precision. However, optical components requiring higher precision have been demanded recently, and birefringence is a big issue in case of, for example, a writable/erasable magneto-optical disc. Birefringence is a property that a light beam passing through a certain substance is separated into two light beams depending on the conditions of polarized light beams. Birefringence index is given as the difference in refractive index between the directions of the polarized lights in the substance.
However, there is a problem that an optical component obtained by molding the common polymeric materials exhibits birefringence. A semiconductor laser beam is used for reading and writing this kind of magneto-optical disc; the existence of an optical component with birefringence in the disc itself or in the lens, for example, on the light path may adversely affect the precision in reading or writing.
Given this factor, Patent Literature 3 proposes a non-birefringent optical resin material containing a polymeric resin and an inorganic fine particle having different signs of birefringence for the purpose of reducing the birefringence. The non-birefringent optical resin material of this proposal is obtained by a method called crystal doping, wherein, for example, a number of inorganic fine particles are dispersed in a polymeric resin, and the bonding chains in the polymeric resin and the inorganic fine particles are oriented in an approximately parallel direction by externally applying a molding force such as drawing so as to diminish the birefringence caused by the orientation of the bonding chain in the polymeric resin with the birefringence of the inorganic fine particles having an opposite sign.
As stated above, inorganic fine particles that may be used for a crystal doping is essential in order to obtain a non-birefringent optical resin material through the crystal doping. The inorganic fine particles are required to have a shape with a high aspect ratio such as a needle- or rod-like shape to align the carbonate particles along the orientation of the bonding chain in the polymeric resin. Carbonate particles which do not influence a light transmittance, which is an important function as an optical material, is demanded. In addition, the average particle size of carbonate crystals need to be sufficiently small compared to the wavelength of a light source to reduce the effect of light scattering by particles as much as possible so that it does not affect the light transmittance.
However, it is difficult to disperse carbonate crystals in a resin without causing flocculation, and merely designing individual carbonate crystal particles to have a desired form or size is not necessarily satisfactory. Therefore, development of a technology for preventing decrease in transmittance of the optical resin material caused by the flocculation of carbonate crystal particles is highly demanded.    Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 59-203728    Patent Literature 2 U.S. Pat. No. 5,164,172    Patent Literature 3 International Publication WO No. 01/25364